The production of multiple sections from a single metal billet was proposed a long time ago as illustrated for example by British patent No. 306, dated Sep. 15, 1910, where a method of production of wire from sheets or strips of soft metals, e.g. lead alloys, tin or soft copper, subjecting said sheets to a rolling and pressing operation by which longitudinal parallel depressions or grooves are formed along the sheet. The individual sections can then be readily manually torn along such grooves or depressions. This early patent discloses a general concept of longitudinally dividing a metal piece into individual sections but was not addressed to the production of steel bars (which present other problems due to the hardness and very high rolling temperatures), nor does it foresee the problems solved by the present invention.
The production of steel bars and particularly rebars for concrete construction is very sensitive to manufacturing costs, because this product is produced massively at low profit margins. Manufacturers continually strive to bring down the operational and capital costs. Consequently, any improvement to the process or equipment which increases the output capacity of a given plant has great commercial value.
A proposal to produce several bars or sections from one billet is shown in U.S. Pat. No 1,977,285 to McCleery dated Oct. 16, 1934. This patent shows a method of rolling metal sections, such as angles, T's or channels in multiples of two or more units (with illustrations of up to eight units, see FIG. 10), all formed at the same time from a bar of a larger size. In the final pass, the sections are separated into individual units by means of rolls arranged to vertically shear and thereby cut or tear the thin web of metal joining the sections, whereby a plurality of units are simultaneously produced thus increasing the capacity of the mill. Although this patent suggests the general idea of preforming several units and then separating the individual units, it teaches to separate the sections by vertically offsetting the inner roll passes of adjacent sections. This patent does not deal with the problem of different tensional stresses between the inner and outer sections, because the sections are rolled following parallel paths without significant separation thereof in the horizontal direction.
Another proposal for simultaneously rolling multiple bars or channels is described in Japanese patent application 61-229402, which description is supplemented by the catalog No. PMD74 dated March 1985 and April 1987 by Nippon Steel Corporation. FIG. 3 of the patent application for example shows a schematic rolling mill for round sections having a rolling process progression as illustrated by FIG. 4. The rolling process starts with a billet (1a), which is pressed to a flat form (1b) in rolling stand 3, subsequently preformed in stand 5 to the form (1d) and then separated longitudinally into four separate sections (1e) in rolling stand 6. It can be seen that the separation of the individual sections is performed by the rolls of stand 6 which is different to the present invention. A better view of the separating rolls is seen on page 2 of the above identified catalog. The main difference between this prior art and the present invention is that the Japanese patent relies only on the rolls with a special shearing and cutting profile to separate the individual sections and not on any other special slitting and splitting devices (see page 1 of the catalog). This patent has the disadvantage that the cutting rolls are more expensive and subject to wearing of the cutting surfaces implying higher operational and maintenance costs.
U.S. Pat. No. 4,193,283 to Bowman et al., dated Mar. 18, 1980 discloses a method and apparatus for slitting billets of metal which are rolled to produce a double or triple stranded bar. The method comprises slitting a multi-stranded bar of "no more than three strands", wherein each strand is longitudinally interconnected to the adjacent strand and is divided into individual sections by diverging the two outer strands. This patent however is expressly limited to only three strands. Although this patent teaches that more than three strands could be handled, it does not teach or suggest to produce a multi-stranded bar of four or more strands, but rather suggests only to apply more than once the same three strand technique in a single mill train. In other words, by applying the invention of this patent a billet would be divided in a first stage into three sections and then each section would be divided in a second stage into three smaller sections, etc. (requiring a much longer line with a greater number of stands). In contrast, the present invention allows production of four or more strands all rolled at the same time (and thus in a shorter distance, a significant capital and operational saving). This patent teaches that preferably all strands are of "the same cross-sectional size and shape." But, if different, then it suggests that the outer strands be identical in shape while the central strand may be different "provided that the percentage reduction in stand C, the slitting stand, is substantially equal for all strands." This patent, being limited to three strands, is mute in respect to a four strand roller and to the preferred shape given to the outer strands, particularly where a uniform product is desired. Thus, one of ordinary skill in the art would not look to this patent for any teaching in how simultaneously to produce more than three separate strands of uniform re-bar.
U.S. Pat. No. 5,284,042 to G. Benedetti, dated Feb. 8, 1994 discloses a method to obtain simultaneously in the cold state a plurality of sections and/or bars, starting with a hot-rolled single multiple-section element, which is then cooled off-line, sheared into final lengths, and then undergoes (assertedly without a break in continuity, but actually in a separate return line, see FIG. 4) an operation in the cold state of simultaneous lengthwise splitting to separate the individual small sections or bars. This patent teaches to cool down the webbed multiple section element before its separation (which operation adds complexity to the bars' manufacture, since the separation is more easily and smoothly done while the steel element is at high temperature). This patent, like all others discussed above, does not teach nor suggest to form the sections having a different cross sectional shape so as to avoid underfilling the roller profile in the subsequent passes of the rolling mill.